1. Field of the Invention
The present invention relates to a position detecting device that detects a position of a detection target based on a detection result of a magnetic field emitted from the detection target.
2. Description of the Related Art
In the technical field of endoscope, a capsule endoscope that can be introduced into a digestive canal of a subject, such as a patient, has conventionally been developed. The capsule endoscope is swallowed from a mouth of the subject to acquire images (hereinafter, occasionally referred to as “in-vivo images”) inside internal organs of the subject while moving in the digestive canal with peristaltic movements, and wirelessly transmits acquired in-vivo images to a receiving device located outside the subject. The capsule endoscope sequentially acquires in-vivo images of the subject since it is introduced into the digestive canal of the subject until it is naturally excreted from the subject.
Furthermore, a system that guides (that is, magnetically guides) a capsule endoscope introduced into a subject by a magnetic force has been proposed (see, for example, International Publication No. WO2005/112733 Pamphlet). In this system, an LC resonance circuit (hereinafter, “LC marker”), which uses a coil and a capacitor, and a magnet are incorporated in the capsule endoscope. The position of the capsule endoscope is detected based on a detection result of an induced magnetic field emitted from the LC marker. The magnetic field formed at the detected position is caused to act on the magnet in the capsule endoscope, thereby magnetically guiding the capsule endoscope in the subject.
A position detecting device that detects the position of a capsule endoscope generally detects, using a plurality of detection coils, an induced magnetic field emitted from an LC marker in the capsule endoscope due to application of an external magnetic field. The position detecting device calculates the position of the capsule endoscope based on a detection result of the induced magnetic field. The position detecting device sets an evaluation function that expresses an error between a field-strength detection value (a measurement value) of each detection coil and a field-strength theoretical value of each detection coil. This field-strength theoretical value is a field-strength value of an induced magnetic field theoretically detected by each detection coil from the LC marker in the capsule endoscope that is in a state of being directed to a provisional direction at a provisional position. The theoretical value is calculated in accordance with a predetermined operation expression. The position detecting device performs optimization convergence calculations based on such an evaluation function to calculate the provisional position and the provisional direction as the position information and direction information of the capsule endoscope. The provisional position and the provisional direction are values at which an error value in the optimization convergence calculation becomes equal to or less than a predetermined threshold (that is, converges). Such a position detecting device repeatedly performs the optimization convergence calculation by using a result of optimization convergence calculation, which is obtained when the error value in the optimization convergence calculation converges to a value equal to or less than a predetermined threshold, as a starting point of calculation for the next optimization convergence calculation. The position detecting device sequentially calculates the position information and direction information of the capsule endoscope.